1. Field of the Invention
The present invention relates to a hot melt ink jet print head.
2. Description of Related Art
As shown in FIG. 1, a conventional print head used in a hot melt ink jet printer (hereinafter referred to as "printer") includes an ink tank PA1, a deaerator PA8, a nozzle head PA5, a compressor PA22, an ink supply unit PA32, and a filter PA33. The ink tank PA1 is made of aluminum alloy using a die-casting method. The ink tank PA1 includes a partition wall which divides the ink tank PA1 into a main chamber PA2 and a sub chamber PA3. The partition wall is formed with a hole serving as a channel PA4. Hot melt ink X (hereinafter referred to as "ink") is stored and melted in the ink tank PA1. The deaerator PA8 includes an air-permeable thin film PA81 and a negative pressure generating device PA82. The deaerator PA8 defines the ink supply channels PA6, PA7. The negative pressure generating device PA82 absorbs air bubbles contained in the ink X. The nozzle head PA5 injects the ink X as an ink droplet onto a recording medium. The nozzle head PA5 is in a fluid communication with the main chamber PA2 via the ink supply channel PA6 and also with the sub chamber PA3 via the ink supply channel PA7.
Internal air pressure of the main chamber PA2 is normally maintained at normal atmospheric pressure, and is increased by a compressor PA22 linked to a pressure regulating opening PA21.
The ink supply unit PA32 supplies the ink X to the sub chamber PA3 through an ink supply opening PA31. The filter PA33 provided near the ink supply opening PA31 removes any foreign matter from the ink X.
A one-way valve PA41 is provided to the channel PA4 and regulates flow of the ink X so that the ink X can flow from the sub chamber PA3 to the main chamber PA2 but not backwards.
A melting point of the ink X is much higher than room temperature. Therefore, when the printer is turned OFF after an printing operation, the ink X starts cooling off and being solidified as decreasing its volume. As a result, air spaces are formed within the ink supply channels PA6, PA7. When the printer is turned ON next time and starts melting the ink X, the air in the ink supply channels PA6, PA7 is trapped and forms air bubbles in the liquid phase ink X. If these air bubbles are ejected along with the ink X from the nozzle head PA5, the ejected ink droplet has a smaller volume than a normal one by a volume of the air bubbles contained in the ejected droplet. As a result, the desired printed image cannot be obtained.
In order to overcome the above-described problems, the air bubbles are removed from the ink supply channels at the start up of the printer. More specifically, the compressor PA22 increases the internal pressure of the main chamber PA2. The ink X is circulated through the ink supply channel PA6, nozzle head PA5, and ink supply channel PA7 and to the sub chamber PA3. In this way, the air bubbles in the ink supply channels PA6, PA7 are collected into the sub chamber PA3.
Then, the printing operation is started. As the print head moves back and forth during the printing operation, an inertial force is generated on the ink X. As a result, the ink X flows from the sub chamber PA3 into the main chamber PA2. Because the one-way valve PA41 prevents the ink X in the main chamber PA2 from returning to the sub chamber PA3, ink level difference h is generated between two chambers PA2, PA3 as shown in FIG. 1. Then, in order to equalize the ink levels, the ink X in the main chamber PA2 slowly flows through the ink supply channel PA6, nozzle head PA5, and ink supply channel PA7 and to the sub chamber PA3.
However, when the ink X is ejected from the nozzle head PA5, a suction force works on the ink supply channels PA6, PA7, thereby the ink X flows in an opposite direction, that is, from the sub chamber PA3 to the nozzle head PA5. Hence, the air bubbles once collected into the sub chamber PA3 are lead to the nozzle head PA5 with the ink X.
In order to prevent the air bubbles from reaching the nozzle head PA5, the deaerator PA8 sucks and removes the air bubbles out of the ink X.